Blood detected in Urinalysis?

I had a Urinalysis done because I was having back pain and abdominal pain. My doctor said everything came back normal except blood was detected in my urine. I asked what the cause was and he said “might have been contaminated”. He told me to come back if my symptoms got worse. I had a bladder and Kidney Ultrasound, which cam back normal as well. Has anyone ever had this happen to them? What did it end up being?